The present invention relates to automatic electronic or "computer" flash systems. In particular, the present invention relates to an improved light sensing device for use with automatic electronic flash systems.
Automatic electronic flash systems include a light producing means, generally a flash tube, which is actuated to illuminate a scene being photographed. A light sensing means detects the scene illumination and produces a light termination or "quench" signal when sufficient light has been produced to properly expose a light sensitive film of an associated camera. The light termination signal actuates a light terminating means which terminates the light flash.
One advantageous flash system utilizes a remote light sensing means or "remote sensor" which is connected to the electronic flash unit by a two-wire cord or cable. Systems of this type are described in U.S. Pat. Nos. 3,714,443 by F. T. Ogawa; 3,737,721 by F. T. Ogawa; 3,793,556 by D. J. Wilwerding; 3,758,822 by D. J. Wilwerding; and 3,914,647 by B. Broekstra and D. J. Wilwerding. While these remote sensors have been generally satisfactory, there has still existed a need for an improved remote sensor which provides a light termination signal even more accurately and reliably and with fewer components than the prior art devices.
In particular, the prior art devices have had some shortcomings. First, in the prior art devices, the termination signal could be, in the worst case, only approximately a five volt level shift. As a result, a very sensitive sensing circuit in the flash unit is required to sense the termination signal. This results in noise sensitivity problems, problems with the maximum number of flash units which can be connected to a single sensing means, problems with maximum shutter cord length, and generally constrained operational requirements.
Second, the prior art devices are susceptible to rate firing of the light activated silicon controlled rectifier (LASCR) when the signal line potential is being established. Since the LASCR has already fired, the light sensing means does not provide the light termination signal when the desired amount of light has been received. Overexposure of the subject, therefore, can result.
Third, when the flash unit includes a correct exposure annunciator, additional circuitry has been required in the remote sensor to eliminate the generation of false termination signals which improperly operate the correct exposure annunciator. The additional components increase the cost of the remote sensor.